1. Field of the Invention
The invention relates to a magnetron sputtering system comprising a chamber, and present within said chamber a flat cathode plate, an anode, an annular auxiliary electrode, which extends axially from a circumferential area of a sputtering surface of said cathode plate, is positioned co-axially with said cathode plate and during operation has a negative potential in relation to the anode potential, and a receiving device for receiving a flat substrate to be coated with cathode plate material at a position opposite and parallel to said cathode plate, and present outside said chamber a magnetic device for generating an electron trap over part of an intermediate area of the sputtering surface of the cathode plate located between a central area and said circumferential area, said device being surrounded by said annular auxiliary electrode.
2. Description of the Prior Art
A magnetron sputtering system of this type is known from European Patent 0 095 211 (the description as well as the claims and the drawing of which patent are considered to be incorporated herein by this reference).
In the known system the cylindrical electrode is a bar-shaped auxiliary electrode, and an annular anode co-axial with the cathode plate is provided, whereby the bar-shaped auxiliary electrode is disposed spaced from the substrate and a device is provided for varying the potential of the annular (tubular) auxiliary electrode in such a manner, that electrons are directed to the annular electrode during operation of the system.
The known system prevents the substrate from being damaged by electrons striking the substrate.
The known system is in particular used for the coating of substrates of temperature-sensitive optical registration carriers having a diameter of about 30 cm.
The operation of the known system will be briefly discussed below by way of supplement to the disclosure of the aforesaid patent.
A magnetron sputtering system distinguishes itself from other sputtering systems by the high rate at which cathode plate material is deposited on a substrate to be coated. This high deposition rate is achieved by an efficient trapping of electrons, as a result of which the gas discharge in the chamber of the magnetron sputtering system is able to produce a strong influx of ions to the cathode plate. This trapping of electrons is efficient because electrons moving away from the cathode plate as a result of the gas discharge are confined in a magnetic field. The magnetic field component parallel to the sputtering surface of the cathode plate, combined with the electric field perpendicular to the dark space on the sputtering surface, forces the electrons into a cycloid orbit, which, as soon as an electron leaves the dark space, changes into a circular orbit in a direction of movement perpendicular to the magnetic field component parallel to the sputtering surface as well as to the electric field. If no collisions take place, the electrons cannot move further away from the sputtering surface than the maximum height of the cycloid orbit or the radius of curvature of the circular orbit. By closing the orbit through which the electrons hop at a short distance from the sputtering surface, the electrons will remain at said short distance from the sputtering surface, unless collisions cause them to move out of the sphere of influence of the magnetic field.
Since the magnetic device is disposed on the cathode plate surface opposite the sputtering surface, the magnetic field is not uniform, which results in a magnetic field component perpendicular to the sputtering surface. More specifically, the magnetic field lines form an arc above the sputtering surface. Neither the perpendicular nor the parallel magnetic field component have a constant value between the point of entry and the point of exit of the magnetic field lines, and the perpendicular field component is reversed. The hopping which is imparted to the electrons under the influence of the parallel magnetic field component takes place in a direction perpendicular to the perpendicular magnetic field component. As a result of that a movement parallel to the sputtering surface is superposed on the hopping. Such a lateral movement is not imparted to electrons which are hopping in an orbit where the perpendicular magnetic field component is reversed, or, in other words, is zero. Electrons which start in another orbit, are subjected to a force in the direction of the orbit where the perpendicular magnetic field component is reversed. They hop and move in zigzag fashion in the closed orbit close to the sputtering surface of the cathode plate.
In the above-described manner a high-density annular plasma is formed close to the sputtering surface of the cathode plate. This plasma does not have a uniform density, however. In those places where the magnetic field component parallel to the sputtering surface becomes weaker, the distance which the electrons are able to move away from the sputtering surface becomes larger and thus the possibility of escape becomes greater. Consequently the erosion of the sputtering surface of the cathode plate does not take place in a uniform fashion. The consumptive use of cathode material is greatest in those places where the magnetic field component perpendicular to the sputtering surface is reversed. This non-uniform use is a drawback of the process of sputtering with a magnetron sputtering system. Further drawbacks resulting therefrom include a shorter life of the cathode plate, increased material costs as a consequence of a change in the plasma impedance which is greater than in the case of a uniform erosion, a reduced deposition rate, because part of the sputtered material is caught in an erosion groove, etc.
The above drawbacks become more serious as the size of the cathode plate decreases, as is for example the case when a smaller substrate is to be coated with cathode plate material, since the magnetic field gradients will increase in that case. After all, a sufficiently strong magnetic field component parallel to the sputtering surface of the cathode plate is required, whilst the distance between the points where the magnetic field intersects the sputtering surface becomes smaller. This implies that the variations in the uniformity of erosion are comparatively greater when smaller cathode plates are used.
In order to solve the above problem a comparatively large cathode plate is used for coating a small substrate so as to obtain the desired operational life of said cathode plate. A drawback of this solution is, however, that a large part of the sputtered cathode plate material does not land on the substrate to be coated, as a consequence of which the chamber needs to be opened frequently in order to remove the material from the cathode plate that has not landed on the substrate.